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Tuesday, June 14, 2011

Analyzing bearing failures

Fig 5.32 (a-c)

Figure 5.32 shows a number of ways in which engine bearings can fail. Generally, these apply to both main bearings and connecting-rod bearings, but some conditions are more likely to occur with connecting- rod bearings.
When analyzing a bearing failure, consider the conditions under which the bearing operates and the loads that the bearing has to carry. This will help to find the cause of the problem.

Lack of oilRefer to Figure 5.32(a). A bearing that is short of oil will overheat and bearing metal will be wiped off the steel back. If the bearing has no oil supply, almost all the bearing metal will melt. The steel back will then run directly on the journal and this will damage the journal. The crankshaft will have to be replaced, or the journals ground undersize. Lack of oil can be due to clogged oil lines, a defective oil pump, a faulty relief valve, or insufficient oil in the oil pan.

Fig 5.32 (d-f)

Fatigue failureRefer to Figure 5.32(b). Repeated application of loads on a bearing will fatigue the bearing metal so that it starts to crack. Small pieces break away from the steel back and small craters or holes form in the bearing. With continued use, more and more particles break away until a large surface of the steel back is exposed.
The bearing is designed to resist fatigue under normal operating conditions, but there are conditions that could cause this problem. One condition is an out- of-round journal, which stresses the bearing with every crankshaft revolution.
If the engine is lugged, the upper half of the connecting-rod bearing will be heavily loaded and could fatigue. High-speed operation could also cause fatigue, but this would be in the lower half of the bearing.

·The terms lugged and lugging refer to operating the engine at low speed at full throttle.

Scratched by dirtRefer to Figure 5.32(c). Some very small particles in the oil could become embedded in the bearing metal. Any larger particles will be carried around with the journal or bearing and will scratch the bearing surface.
The cause could be poor cleaning during repairs, contaminants introduced with the oil, or a faulty oil filter.

Tapered journaLRefer to Figure 5.32(d). If a journal is tapered, one side of the bearing will carry most of the load. This side will overheat and lose its bearing metal.

This should not be confused with a failure from a bent connecting rod. With a tapered journal, both halves of the bearing will fail on the same side. With a bent connecting rod, the top half of the bearing will fail on one side and the bottom half will fail on the opposite side.

Radius rideRefer to Figure 5.32(e). If the radius between the crankshaft journal and web is too large, the edge of the bearing will ride on the radius. There would be no clearance and metal could be wiped off the edge of the bearing. Also, the bearing could be cramped, causing poor seating, rapid fatigue and early failure.
This problem could occur after a crankshaft has been ground if the edges of the journals have not been given the correct radius.

Poor seatingRefer to Figure 5.32(f). Poor seating of a bearing in its bore will cause high spots where there is not enough bearing clearance. This will show up as bright sections where the bearing is worn.
Poor seating not only reduces bearing clearance and causes wear, but it leaves air space between the back of the bearing shell and its bore. This interferes with heat transfer from the bearing to its cap or to the crankcase.